1. Field of the Invention
The present invention relates to a laser apparatus and a method of driving a diffraction grating.
2. Description of the Related Art
Since a laser apparatus has features of small size, low power consumption, and so forth, it has been widely used for information devices in recent years. For example, in a holographic data storage (HDS), data are recorded using interference of two beams that are split from one beam by a beam splitter and combined again on a disc.
As a light source for holographic record and reproduction, a single mode laser light source is used. For example, a gas laser or a second harmonic generation (SHG) laser is used. Instead, an external cavity type semiconductor type laser using a laser diode (LD) can be used.
A regular laser diode emits multi mode light. Thus, it does not provide sufficient coherency. However, when the laser diode is structured as an external cavity type, it emits single mode light. As a result, a light source for a hologram record and reproduction having high coherency can be accomplished. A typical structure of the laser apparatus having an external cavity type semiconductor laser is described in the following non-patent document.
[Non-Patent Document 1]
L. Ricci, et al. “A compact grating-stabilized diode laser system for atomic physics,” Optics Communications, 117 1995, pp 541-549
FIG. 1 shows a structure of an external cavity type semiconductor laser called Littrow type using a laser diode. Laser light emitted from a laser diode 1 is collimated by a collimator lens 2. The collimated light enters a reflection type diffraction grating (hereinafter referred to as the grating) 3.
The grating 3 is mounted on a grating mounting section 4. The grating mounting section 4 is held on a stay 6 through a leaf spring 5. One end portion and another end portion on one side of the grating mounting section 4 are secured to the leaf spring 5 and a screw 7, respectively. By turning the screw 7, the other end portion secured thereto is moved in the upper and lower directions. As a result, the angle of the grating 3 is varied. The screw 7 is inserted into a screw support (not shown).
When the laser light is reflected by the grating 3, the laser light is split into zero-th order light L0 and first order lights L1a, L1b, and L1c. The reflection angles of the first order lights vary corresponding to the wavelengths thereof. The first order lights L1a, L1b, and L1c have different wavelengths.
The first order light L1a having a wavelength corresponding to the angle of the grating 3 is retro-injected into the laser diode 1 through the collimator lens 2. As a result, the wavelength of the first order light L1a retro-injected into the laser diode 1 becomes dominant. Thus, the laser diode 1 emits single mode light. The emitted single mode light has a wavelength that is the same as that of light returned from the grating 3. In other words, the first order light L1a causes the grating 3 and the laser diode 1 to form a cavity therebetween. The laser diode 1 oscillates with a wavelength that depends on the shape of the grating 3 and the distance between the grating 3 and the laser diode 1. The grating 3 reflects the zero-th order light L0 and emits it to the outside like a mirror. The zero-th order light L0 is used to hologram record and reproduce data.
In the external cavity type laser apparatus, there are an external cavity mode hop region and a laser chip mode hop region. In the external cavity mode hop region, as the laser power increases, the wavelength of laser light gradually increases. In the laser chip mode hop region, as the laser power increases, the wavelength of emitted laser light sharply decreases. The wavelength of the laser light discretely shifts as the laser power increases.
As the relationship between the laser power and wavelength of laser light outputted from the laser apparatus, assuming that the laser power and the wavelength are plotted on the horizontal axis and vertical axis, respectively, on a graph, as the laser power of the laser light increases, the wavelength of the laser light varies in a saw wave shape. The wavelength of the laser diode varies for around 0.04 nm in mode hops of the laser diode in the relationship between the power and wavelength. As shown in FIG. 2, with a predetermined laser power, laser light having only a predetermined wavelength can be generated. In the example shown in FIG. 2, laser light having a wavelength (407.89 nm) is denoted.
As shown in FIG. 3, in a tunable laser apparatus, by changing the angle of the grating 3, the wavelength is varied by for example ±2 nm to the center wavelength for example 403.5 nm. As will be described later, (A−B)/(A+B) denotes a calculated result of signals A and B of which the position of a spot that varies corresponding to the wavelength of the laser light is detected by a two-divided detector.
For example, the tunable laser apparatus is set so that the calculated output becomes 0 at the center wavelength. When data are recorded on a hologram medium by the wavelength multiplexing method, it is necessary to satisfy a condition of which the wavelength variation step is around 100 pm. This wavelength variation step corresponds to a rotation angle variation step of 0.015° in the grating 3.
However, when the angle of the grating 3 is changed, the direction of the zero-th order light L0 that is emitted varies. Thus, in this case, the laser apparatus may not be used as a light source. The following non-patent document 2 describes a structure of a mirror set that allows exit light to take the same optical path as incident light even if the angle of the grating 3 is changed.
[Non-Patent Document 2]
T. M. Hard, “Laser Wavelength Selection and Output Coupling by a Grating,” APPLIED OPTICS, Vol. 9, No. 8, August 1970, pp 1825-1830
FIG. 4 shows a laser apparatus having a structure of a mirror set. A grating 3 and a mirror 8 are countered with an open angle of for example 90°. Laser light emitted from a laser diode 1 is reflected by the grating 3. In addition, the reflected light is reflected by the mirror 8 and then exited to the outside. The grating 3 and the mirror 8 are rotated around a rotation axis 9 while the open angle is kept. The rotation axis 9 is disposed on the other end of the grating 3. The rotation axis 9 is perpendicular to the optical axis of the laser light emitted form the laser diode 1. In addition, the rotation axis 9 extends along grooves of the grating 3.
The exit direction of the zero-th order light L0 in the case that the grating 3 and the mirror 8 are positioned as denoted by solid arrow lines is different from that in the case that they are positioned as denoted by dotted arrow lines. When the exit direction of the zero-th order light L0 of the laser apparatus varies, it is not suitable to use the laser apparatus as a light source.
When the grating 3 and the mirror 8 in the structure described in the foregoing non-patent document 2 is rotated, the direction of the laser light that is emitted does not vary. Next, with reference to FIG. 5, this theory will be described. One end of the grating 3 and one end of the mirror 8 are connected at an intersection of an extended line of a reflection surface of the grating 3 and an extended line of a reflection surface of the mirror 8 by a rotation axis 11 in parallel with an extended direction of grooves of the grating 3. The rotation axis 11 is the center of a circle 12. The angle of the reflection surface of the grating 3 and the reflection surface of the mirror 8 is denoted by V.
When predetermined incident light 13 is given from point c to point d, the incident light 13 enters the grating 3 at point d. Zero-th order light 14 is reflected at the same angle as the incident angle. The reflected light travels to point e. At point e, the mirror 8 receives the zero-th order light 14 and emits reflected light 15 to point f. An extended line of the incident light 13 and an extended line of the reflected light 15 are intersected at point j. The extended lines and the line of the incident light 13 are tangents of the circle 12.
When the grating 3 and the mirror 8 are rotated around the rotation axis 11 while the angle V is kept, the grating 3 and the mirror 8 are moved to positions denoted by dotted lines. At this point, the predetermined incident light 13 extends from point c to point g. At point g, the incident light 13 enters the grating 3. The grating 3 emits zero-th order light 16. The zero-th order light 16 extends from point g to point h. At point h, the zero-th order light 16 is reflected by the mirror 8. The reflected light 15 extends from point h to point f.
After the grating 3 and the mirror 8 are rotated, the extended line of the incident light 13, the extended line of the reflected light 15, and the line of the zero-th order light 16 are tangents of the circle 12. Thus, when the grating 3 and the mirror 8 are rotated with a fulcrum of the intersection of the extended lines of the reflection surfaces, it is clear that the angle of the predetermined incident light 13 and the reflected light 15 is kept as a constant value W.
When this theory is applied, even if the inclination of the grating 3 is changed to vary the wavelength of laser light, the output laser light can be exited to a predetermined position. As a result, a tunable laser can be accomplished.
The rotation angle at which the grating 3 and the mirror 8 having the structure of the mirror set are rotated while the opposite angle is kept is as small as 0.0150 per step. As a device that accomplishes a fine rotation angle, a stepping motor is known.